In the old days of colorizing film was done BY HAND. Literally painting the negatives and reshooting it to make a new frame. While color film was expensive, it wasn't for still images and records of the colors used in fabrics, and set pieces allowed them to pic the color they needed.
Now it's digital, software analyzes the gray in the movie and assigns a pixel a certain degree based on light level and darkness. Thus a color hex pixel is assigned.
